make CommandStart return a StartMessage

The goal is to be able to run CommandStart in the main thread when -J is
used, rather than unncessarily passing it off to a worker thread, which
incurs overhead that is signficant when the CommandStart is going to
quickly decide to stop.

To do that, the message it displays needs to be displayed in the worker
thread, after the CommandStart has run.

Also, the change will mean that CommandStart will no longer necessarily
run with the same Annex state as CommandPerform. While its docs already
said it should avoid modifying Annex state, I audited all the
CommandStart code as part of the conversion. (Note that CommandSeek
already sometimes runs with a different Annex state, and that has not been
a source of any problems, so I am not too worried that this change will
lead to breakage going forward.)

The only modification of Annex state I found was it calling
allowMessages in some Commands that default to noMessages. Dealt with
that by adding a startCustomOutput and a startingUsualMessages.
This lets a command start with noMessages and then select the output it
wants for each CommandStart.

One bit of breakage: onlyActionOn has been removed from commands that used it.
The plan is that, since a StartMessage contains an ActionItem,
when a Key can be extracted from that, the parallel job runner can
run onlyActionOn' automatically. Then commands won't need to worry about
this detail. Future work.

Otherwise, this was a fairly straightforward process of making each
CommandStart compile again. Hopefully other behavior changes were mostly
avoided.

In a few cases, a command had a CommandStart that called a CommandPerform
that then called showStart multiple times. I have collapsed those
down to a single start action. The main command to perhaps suffer from it
is Command.Direct, which used to show a start for each file, and no
longer does.

Another minor behavior change is that some commands used showStart
before, but had an associated file and a Key available, so were changed
to ShowStart with an ActionItemAssociatedFile. That will not change the
normal output or behavior, but --json output will now include the key.
This should not break it for anyone using a real json parser.

Command/Move.hs

@@ -74,7 +74,7 @@ startKey fromto removewhen =
 	uncurry $ start' fromto removewhen (AssociatedFile Nothing)
 
 start' :: FromToHereOptions -> RemoveWhen -> AssociatedFile -> Key -> ActionItem -> CommandStart
-start' fromto removewhen afile key ai = onlyActionOn key $
+start' fromto removewhen afile key ai =
 	case fromto of
 		Right (FromRemote src) ->
 			checkFailedTransferDirection ai Download $
@@ -86,9 +86,9 @@ start' fromto removewhen afile key ai = onlyActionOn key $
 			checkFailedTransferDirection ai Download $
 				toHereStart removewhen afile key ai
 
-showMoveAction :: RemoveWhen -> Key -> ActionItem -> Annex ()
-showMoveAction RemoveNever = showStartKey "copy"
-showMoveAction _ = showStartKey "move"
+describeMoveAction :: RemoveWhen -> String
+describeMoveAction RemoveNever = "copy"
+describeMoveAction _ = "move"
 
 toStart :: RemoveWhen -> AssociatedFile -> Key -> ActionItem -> Remote -> CommandStart
 toStart removewhen afile key ai dest = do
@@ -108,9 +108,8 @@ toStart' dest removewhen afile key ai = do
 			)
 		else go False (Remote.hasKey dest key)
   where
-	go fastcheck isthere = do
-		showMoveAction removewhen key ai
-		next $ toPerform dest removewhen key afile fastcheck =<< isthere
+	go fastcheck isthere = starting (describeMoveAction removewhen) ai $
+		toPerform dest removewhen key afile fastcheck =<< isthere
 
 expectedPresent :: Remote -> Key -> Annex Bool
 expectedPresent dest key = do
@@ -182,9 +181,9 @@ fromStart removewhen afile key ai src = case removewhen of
 	RemoveNever -> stopUnless (not <$> inAnnex key) go
 	RemoveSafe -> go
   where
-	go = stopUnless (fromOk src key) $ do
-		showMoveAction removewhen key ai
-		next $ fromPerform src removewhen key afile
+	go = stopUnless (fromOk src key) $
+		starting (describeMoveAction removewhen) ai $
+			fromPerform src removewhen key afile
 
 fromOk :: Remote -> Key -> Annex Bool
 fromOk src key 
@@ -250,9 +249,9 @@ toHereStart removewhen afile key ai = case removewhen of
 	go = do
 		rs <- Remote.keyPossibilities key
 		forM_ rs $ \r ->
-			includeCommandAction $ do
-				showMoveAction removewhen key ai
-				next $ fromPerform r removewhen key afile
+			includeCommandAction $
+				starting (describeMoveAction removewhen) ai $
+					fromPerform r removewhen key afile
 		stop
 
 {- The goal of this command is to allow the user maximum freedom to move